FIG. 9 is a circuit block diagram of conventional power supply apparatus 3 and electronic device 2 disclosed in Japanese Patent Laid-Open Publication No. 2002-291172. Battery 1 outputs a voltage ranging from 3.0V to 4.2V. A negative electrode of battery 1 is connected to a ground, and a positive electrode of battery 1 is connected to mobile phone 2, an electronic device, and input terminal 4 of power supply apparatus 3.
Integrated circuits (ICs) used for the electronic device having fine patterns have a small power source voltage ranging from 1.2V to 1.8V. In order to supply the voltage, power supply apparatus 3 converts an output voltage of battery 1 into the voltage ranging from 1.2V to 1.8V. A large current flows in mobile phone 2, and accordingly adds a ripple to a power source applied to input terminal 4. Power supply apparatus 3 removes the ripple in order to supply a power source to a circuit, such as an analog circuit, which is easily influenced by the ripple.
Power supply apparatus 3 includes DC-DC converter 5 for converting a power source voltage applied to input terminal 4 to a voltage of 1.2V, output terminal 6 connected to an output of DC-DC converter 5, regulator 7 for converting the power source voltage applied to input terminal 4 to a voltage of 1.2V, filter 8 connected to an output of regulator 7 to remove a ripple in the output, output terminal 9 connected to an output of filter 8, regulator 10 for converting the power source voltage applied to input terminal 4 to a voltage of 1.8V or 2.8V; and output terminal 11 connected to an output of regulator 10. Filter 8 further removes a ripple that has not been removed by regulator 7, and supplies a more stable voltage to output terminal 9.
Output terminal 9 is connected with analog circuit 13 that is influenced most easily by noises. Output terminal 11 is connected with high-frequency circuit 14 that is influenced next most easily by the noises. Output terminal 6 is connected with logic circuit 12 that is not relatively influenced by the noises.
DC-DC converter 5 converts a voltage, and simultaneously generates high-frequency noises. However, DC-DC converter 5 has a high conversion efficiency, and supplies a power source preferably to logic circuit 12 that is not relatively influenced by the noises, thus reducing a power consumption.
Analog circuit 13 and high-frequency circuit 14 are influenced easily by the noises, and regulators 7 and 10 that do not generate noises supply a power source to circuits 13 and 14. Analog circuit 13 is influenced by the noises, and filter 8 reduces the noises.
Conventional power supply apparatus 3 includes regulators 7 and 10 having high power consumption, thus not being suitable for reducing power. In FIG. 10, battery 1 outputs a voltage of 3.6V. A current of 20 mA flows in regulator 7 and analog circuit 13. A voltage of 1.2V is applied to analog circuit 13. Regulator 7 has an input port and an output port which have a potential difference of 2.4V(=3.6V−1.2V) between the ports. Analog circuit 13 consumes a power of 24 mW while regulator 7 itself consumes a power of 48 mW, which is larger than the power consumed by analog circuit 13.